There is a known gas sensor which includes a metal oxide semiconductor such as tin oxide (SnO2), supporting noble metal such as platinum (Pt) as a catalyst and which is arranged to sense a concentration change of a measurement gas to be sensed by utilizing an electric characteristic (such as resistance) varied by the measurement gas. In a production method of such a gas sensor, the noble metal is supported in a dispersed state on the surface of the metal oxide semiconductor by calcination after impregnation of metal oxide semiconductor powder in a liquid solution containing the noble metal element (cf. patent document 1, for example).
It is known to achieve a high gas sensitivity to various smells (bad smells, especially) considered to be caused by hydrogen sulfide or mercaptans by the use of a gas sensing layer including a basic metal oxide supported, as catalyst, on a metal oxide semiconductor (cf. patent document 2, for example). However, the basic metal oxide is high in electric resistance, and the structure of the basic metal oxide supported on the metal oxide semiconductor powder individually as in the patent document 1 increases the electric resistance of the gas sensing layer, and makes difficult the circuit design of the gas sensor. It is possible to prevent an increase of the electric resistance of the gas sensing layer with the structure in which the basic metal oxide is supported on the surface of sintered product (gas sensing layer) obtained by sintering the gas sensing layer of metal oxide semiconductor powder (cf. a patent document 3, for example).
The gas sensing layer of such a gas sensor does not react with the measurement gas at normal temperature, and the gas sensing layer becomes activated and reactive to the measurement gas when heated to 200˜400° C. Therefore, in general, a heat generating resistor is provided in a base member such as a semiconductor substrate on which the gas sensing layer is formed. However, when the gas sensor is driven at high temperatures by using the heat generating resistor, there is a possibility of separation at the interface between the gas sensing layer and the base member due to a difference in thermal expansion between the gas sensing layer and base member. Moreover, in such a gas sensor, there is a natural demand for increasing the mechanical adhesive strength between the gas sensing layer and base member to obtain a high reliability. Therefore, there are proposed various structures having an adhesion layer formed between the gas sensing layer and the base member. One example proposes a heat type sensor arranged to mitigate heat expansion with a hafnium oxide layer formed between a heater of platinum (Pt) in the form of a thin film resistor and an underlying insulating film (cf. a patent document 4, for example). Another example proposes a gas sensor arranged to mitigate a thermal expansion coefficient by increasing the surface area of an electrode layer by producing concavities and convexities in the surface of the electrode layer on the substrate, and further by providing, between this electrode layer and the gas sensing layer, an intermediate layer of an electrically conductive functionally graded material in which the compositions of the materials of both layers are varied gradually (cf. a patent document 5).    Patent Document 1: JP 63-279150 (A)    Patent Document 2: JP 06-27719 (B)    Patent Document 3: JP 05-51096 (B)    Patent Document 4: JP 2001-91486 (A)    Patent Document 5: JP 09-33470 (A)